1. Field of the Invention
This invention relates to a combustion state control system of an internal combustion engine, more particularly to a system for controlling combustion state of a direct injection spark ignition multi-cylinder engine, in which gasoline fuel is directly injected into the combustion chamber of the engine.
2. Description of the Related Art
Japanese Laid-Open Patent Application No. Hei 9 (1997) -268942 teaches, when the combustion state degrades or deteriorates in a direct injection spark ignition engine operable in a stratified combustion operation mode (a kind of lean burn operation mode), suppressing the combustion state degradation by changing gradually at least one of the Exhaust Gas Recirculation flow rate (or amount), the ignition timing and the air/fuel ratio.
Specifically, it teaches suppressing the combustion state degradation, at first, through the ignition timing or the air/fuel ratio, and if the combustion state remains unchanged, then decreasing the EGR flow rate. If the combustion still remains at the degraded state, it teaches changing the fuel injection timing from the compression stroke to the intake stroke, and if the situation nevertheless remains unchanged, enriching the air/fuel ratio up to the stoichiometric air/fuel ratio.
Thus, this prior art discloses changing various engine operation parameters when the combustion state is found to be degraded. However, this prior art discusses the problem under this stratified combustion operation mode only as an example of the lean burn operation mode.
However, some engines are operable in a plurality of lean-burn operation modes, e.g. two modes comprising this stratified combustion operation mode (diffusion combustion operation mode) in which the desired air/fuel ratio KCMD is determined to be, at maximum 60.0:1 and fuel is injected at the compression stroke, and a pre-mixture combustion operation mode (uniform combustion operation mode) in which the desired air/fuel ratio KCMD is determined to be richer (less lean) than that of the stratified operation mode and fuel is injected at the intake stroke.
In an engine having these two kinds of lean-burn operation modes, the parameters contributing to the combustion stability are not always common for the two modes. Specifically, the air/fuel ratio, the EGR flow rate and the purge flow rate (of vaporized fuel to the air intake system) play a significant role in stabilizing the combustion state in the stratified combustion operation mode, while the air/fuel ratio, the EGR flow rate and the ignition timing exhibits excellent contribution to stabilize the combustion state in the pre-mixture combustion operation mode.
Accordingly, it is preferable to select the parameters most effective in restoring the combustion state stability, if it degrades, in response to the selected operation mode and to suppress the combustion state degradation by changing the selected parameters. However, the aforesaid prior art teaches or suggests nothing on this.
An object of the present invention is therefore to provide a combustion state control system of an internal combustion engine operable in a selected one of a plurality of lean-burn operation modes, which selects the parameters most effective in restoring the combustion state stability, if it degrades, in response to the selected operation mode and suppresses the combustion state degradation by changing the selected parameters.
This invention achieves this object by providing a system for controlling combustion state of an internal combustion engine in which gasoline fuel is directly injected into cylinder combustion chambers, including: an air/fuel ratio sensor for detecting an air/fuel ratio of exhaust gas exhausted from the engine; engine operating condition detecting module which detects operating conditions of the engine including at least an engine speed and an engine load; fuel injection amount calculating module which calculates a basic fuel injection amount based on at least the engine speed and the engine load of the detected engine operating conditions and corrects the basic fuel injection amount, such that an error between the detected air/fuel ratio and a desired air/fuel ratio decreases, to determine an output fuel injection amount to be injected into the cylinder combustion chambers of the engine; ignition timing calculating module which calculates an ignition timing based on at least the engine speed and the engine load of the detected engine operating conditions to ignite a mixture of the injected fuel and air drawn through an air intake system and flowing into the cylinder combustion chambers; EGR flow rate regulating module which regulates an EGR flow rate of the exhaust gas recirculated through a pipe connecting an exhaust system and an air intake system of the engine; and combustion state control module which conducts a control to suppress degradation of combustion state, if it is determined to be degraded, by changing at least one of the desired air/fuel ratio, the EGR flow rate and the ignition timing when the engine is operated in a lean-burn operation mode of a stratified combustion operation mode in which the desired air/fuel ratio is set to be a lean value. The characteristic features are that the system includes; purge flow rate regulating module which regulates a purge flow rate of vaporized fuel to be purged through a pipe connecting a canister and the air intake system of the engine; and operation mode selecting module which selects one of the lean-burn operation modes comprising the stratified combustion operation mode and a pre-mixture combustion operation mode in which the desired air/fuel ratio is set to be a value less lean than that of the stratified combustion mode; and the combustion state control module conducts the control to suppress the degradation of combustion state, if it is determined to degraded, by changing the desired air/fuel ratio, the EGR flow rate and the ignition timing when the engine is operated in the stratified combustion operation mode, while by changing the desired air/fuel ratio, the EGR flow rate and the purge flow rate when the engine is operated in the pre-mixture combustion operation mode.